It will take a long time for them to dry if they are large unless you cut them up. I would suggest cutting it into slabs between 1 and 2 inches thick (or thicker if you need larger pieces) and sealing them all around with wax or green wood selaer (Anchorseal is one brand). You need to seal burls allaround because the swirling grain makes them more prone to crack anywhere, not just on the end grain.
You need to make sure you dry them slowly as burls are prone to twisting and cracking if dried too fast. Make sure to cut the pieces a little thicker than you need to allow for some wood loss due to shrinkage and twisting/cracking.